1. Field of the Invention
This invention relates to heat storage apparatus and, more particularly, to a heat storage apparatus and method utilizing a fusible salt as the heat storage medium.
2. The Prior Art
Conventionally, thermal energy is stored as a temperature rise in a chemically inert medium such as water, metal, rocks and the like. Temperature rise for water is limited by the boiling point of the water. Heat storage at higher temperatures, particularly for solar collector applications, becomes less efficient and more costly because of equipment costs and energy losses from storage and during transfer.
Historically, water is usually considered to be the most suitable material for thermal energy storage primarily because it is inexpensive and has a reasonably high heat capacity. In particular, water has about the highest heat capacity per kilogram per dollar of any oridinary material. However, water requires containment tanks which, in sizes large enough to hold tons of water, are relatively expensive. While the heat conductance of water is rather low, temperature differentials in the water cause the colder water to sink and the warmer water to rise. This creates a relatively rapid internal circulation which transfers heat in spite of the relatively low thermal conductance of water.
Other heat storage systems involve pebble beds or rock piles. The large surface area and tortuous path through the pebble bed insures a very rapid heat exchange. Conduction of heat through the pebble bed itself, with 1/3 of its volume occupied by air spaces between the pebbles, is relatively low because the pebbles have limited areas of contact for heat transfer. The loss of heat through the containing walls to the surrounding atmosphere is thus much reduced. The pebbles also reduce the thermal circulation of the enclosed air by entrapment within the voids. These vivid spaces in a pebble bed further reduce the effective heat storage volume. While the heat capacity of rock is considerably less than that of water, the density of rock is much greater. A cubic foot of solid rock stores about 9.0 kcal/.degree.C. whereas a cubic foot of water stores about 28.3 kcal/.degree. C.
For some special cases, the heat capacity and heat conductance of metals is very good and metals may thus be used for storing heat. On the basis of weight, the heat storage of metals is only 1/10 (iron) and 1/4 (aluminum) as much as that of water. However, on the basis of volume the difference is much less because the metal densities are greater although the cost of heat storage by metals is also much greater than by water or rocks.
Heat storage by raising the temperature of a chemically inert material involves the problems of (a) volume of material (b) cost of material, (c) heat transfer to and from the storage unit, and (d) thermal insulation. The latter consideration is important since heat losses occur (1) by conduction to structural materials in contact with the heat-storing substance, (2) by convection in air particularly in windy environments, and (3) by radiation in the infrared. All of these heat losses dictate that the heat-storage unit have as small an external surface area as possible and that it be protected with adequate insulation.
Changes in the physical state and chemical reactions involve much greater thermal effects than temperature changes of inert materials alone. Accordingly, heat storage vessels for chemical reactants can be smaller in size and less expensive. Moreover the operative temperatures may be lower and the operative range narrower so that the cost of insulation is reduced correspondingly. Additionally, some suitable chemicals are relatively inexpensive.
Importantly, the chemical reaction must be easily reversible over a range of temperatures that is not too large. As in many practical chemical operations it is not sufficient to meet the thermodynamic requirements; it is also necessary to have the kinetic reaction favorable so that the reaction will proceed rapidly enough. The simplest chemical heat storage system involves the transition between a solid phase and a liquid phase. Salt hydrates involve this phase change and are among the simplest types of chemicals used for heat storage. A good example of a salt hydrate is sodium sulfate in its transition between the hydrated and the unhydrated crystals: EQU Na.sub.2 SO.sub.4.10H.sub.2 O .revreaction.Na.sub.2 SO.sub.4 +10H.sub.2 O
When the temperature is raised above 32.3.degree. C. a concentrated solution of the anhydrous salt is formed with the absorption of heat. When the temperature falls below 32.3.degree. C. the anhydrous salt reacts with the water and evolves heat. The heat of reaction is about 50 calories per gram of hydrated salt.
However, after many cycles there is a tendancy to develop supersaturated solutions rather than heat-evolving crystallization. Additionally, the crystals tend to settle to the bottom of the container. The resultant stratification of the salt interferes with the reversibility of the transition. The rate of crystallization for sodium sulfate in this environment is about 1.25 cm per hour which sets a limit to the rate of heat evolution.
In view of the foregoing it would be an advancement in the art to provide an apparatus and method whereby improvements are made in the apparatus and method for the chemical storage of heat and which is particularly suitable for use with a solar collector system. Such an apparatus and method is claimed herein.